Effect of a CaO–SiO2–Al2O3 Composite Early-Strength Agent on the Early Mechanical Properties of Cement-Based Repair Materials

Abstract

The early mechanical performance of cement-based repair materials is critical for the rapid rehabilitation of concrete structures and tunnel anchorage engineering. In this study, a reference mixture (ROA) without any accelerator, a general early-strength agent (GEA) based on aluminum sulfate, and a composite early-strength agent (CEA) composed of CaO–SiO2–Al2O3 were comparatively investigated. The effects on setting time, compressive strength, flexural bond strength, and early drying shrinkage were systematically evaluated. The degree of hydration (DoH) was determined by the chemically bound water method, and the microstructure was characterized by scanning electron microscopy (SEM). The results show that CEA significantly shortened the setting time, with a final setting time of only 9.5 min, representing a 55.2% reduction compared with ROA. At 4 h, the compressive strength of CEA was 5.2% and 16.7% higher than those of GEA and ROA, respectively; at 1 d, the corresponding increases were 6.8% and 25.0%; and no strength regression was observed at 28 d. The 1 d flexural bond strength of CEA reached 6.85 MPa, 62.8% higher than that of ROA. Moreover, CEA effectively reduced early drying shrinkage, with the 7 d shrinkage rate being 18.6% lower than that of ROA. The DoH of CEA at 1 d reached 68.5%, 32.3 percentage points higher than that of ROA. SEM observations revealed that CEA promoted the formation of needle-like ettringite (AFt) crystals that interlocked in a random three-dimensional network, with calcium silicate hydrate (C–S–H) gel filling the interstices, resulting in a dense microstructure. These findings provide a systematic theoretical basis for the design of high-performance cement-based repair materials with composite early-strength agents.